I heard something odd today. I was told by a camera assistant that he was working with a DP last week who instead of changing lenses just changed from 4K to 2 K on the Epic to crop his shot so he could get the right composition.

Is that OK just to jump around like that instead of changing the lense to your desired composition.

Regarding your question, I dunno, that sounds very strange to me, but it's possible to do that. In terms of going from 4k to 2k, the resolution does crop down, so I suppose in essence you can alter your composition changing your resolution.

I'd just stick to either one of the resolutions depending on what your project doing, so for example if you have 35mm pl lenses, just stick to 4k but if you got s16mm lenses go 2k, as i'm sure your aware.

I think it's a bit lazy if you already have the lenses -- you really should avoid 2K on the Red Epic except when there is no other alternative, like for extreme slow-motion. At 2K, you are enlarging noise artifacts plus losing resolution (obviously.) Also 2K Raw doesn't make for great HD RGB, you are under-sampled on your red and blue information. I mean, if this was a situation where for a documentary shooting quickly it was faster to change to 2K from 4K rather than miss the shot by changing a lens, but on the other hand, you'd probably use a zoom in that situation anyway, plus it doesn't take that much time to switch a prime lens.

Why would cropping to 2K get you the "right composition" as opposed to using a lens that was twice as long???

Also 2K Raw doesn't make for great HD RGB, you are under-sampled on your red and blue information.

I was going to ask about this. In theory, 2K down sampled from 4K has more color information per pixel (and fewer or less severe artifacts), so would there be a visible difference between the two, viewing at 2K or 1080?

It's mainly just a resolution issue, you'll get more from converting 4K raw to 1080P RGB than from converting from 2K raw. I don't think it's a matter of getting "better" colors one way or the other, just better resolution in red and blue areas of the image if ending up at 1080P RGB.